Cotext

It is difficult to find a meaning that will work in each of
the occurrences:

My lord was standing beside an 'anak
wall,
with 'anak in his hand.
8 And Adonai said to me,
"What do you see, Amos?"
And I said,
"'anak."
Then my lord said,
"See, I am putting 'anak
in the middle of my people Israel

The best possibility is the understand 'anak as
"lead" with the idea of "plumbline", in which case "a
wall of 'anak" might be a wall built plumb square. However, there
is a word mishqolet which appears twice with the meaning "plumbline";
and the cognates do not support "lead".

cognate languages

Although some claims have been made that the parallel Acadian
word (anaku) can mean "lead", tin is clearly
the more likely meaning.

early versions

The variety of renderings of אֲנָךְ 'anak among
the early translators shows that the word was already problematic:

Targum (Aramaic) = "judgment"

LXX (Greek) "steel, diamond"

Theodotion (Greek) = "molten"

Aquila (Greek) = "shining"

Vulgate (Latin) = "plaster"

an original suggestion

Andersen & Freedman (756-759) have interesting
suggestions which enable the similarity of construction of this vision and
the next to be highlighted, they claim three different meanings for אֲנָךְ 'anak. In
late Hebrew both "plastered" (cf. Vulgate) and "grief, wrong,
oppression" are attested, while for the central two uses they follow
the Acadian "tin":

My Lord was standing beside a plastered wall,
a lump of tin in his hand. ... "What do
you see Amos?" I said, "A lump of tin." ... "I am putting grief
among my people Israel..."